1. Field of the Invention
The present invention relates to chemical treatment of soils and construction materials for the construction of foundation or other concrete structures, and paved or unpaved surface to prevent or remedy distresses/damage of infrastructures due to the formation of ettringite or similar minerals.
2. Description of the Related Art
Existing expansive soils in the construction sites are always the concerns for the stability of the subgrade, base, and subbase, causing the distress and damage of foundations, paved or unpaved road surfaces and airport runways, etc. The formation of undesirable ettringite [Ca6Al2(SO4)3(OH)12-26H2O, Hydrated Calcium Aluminum Sulfate Hydroxide] or similar minerals is known to cause distresses and failures in natural and construction materials, such as examples above and in concrete structures. Phenomenon like heave in lime-stabilized sulfate-rich soils is well known to be due to the formation of ettringite, in which process a large amount of water is combined in the crystal lattice resulting in local expansion. One important mechanism of sulfate attack to portland cement concrete is also the formation of expansive ettringite.
The use of lime as soil stabilizer has been very popular in construction practice. To deal with the distresses associated with ettringite formation or similar minerals in lime-stabilized soils, two general approaches are used in current practice. The first approach is to cut or limit the supply of water, sulfate and/or aluminum in the soil. The second approach is to allow a mellowing period in which ettringite is allowed to form for a period of from 24 hours to several days, followed by the compaction in soil stabilization.
The first approach is often used in stabilizing expansive soil with moderate sulfate content. A few examples are given below. Polymer stabilizers, as an example, are purposed to coat individual soil particles and thus cut the supply of water. The depletion of available water is aimed to prevent the formation of ettringite or similar minerals. The use of gypsum, blast furnace slag, and portland type cement is another example of solidifying high plasticity soils. More recently, soil stabilization uses fly ash and ground granulated blast furnace slag (GGBFS) to change the Al/Si ratio and thus reduce the stability field of ettringite. The direct use of lime is known to cause more swelling than the original soil if the sulfate concentration is more than 3000 ppm (TxDOT 2005).
The second approach is used when the sulfate content of soil is between 3000 and 8000 ppm. Soil is mixed with lime and water first. The mixture is left alone for a period of from 24 hours to several days and then compacted. It is assumed that during the mellowing process most ettringite or similar minerals will form and its expansion will, therefore, not affect the soil compacted later. This approach of waiting is, however, costly for construction, and not applicable to soil with sulfate content larger than 8000 ppm.
When soil has the sulfate content larger than 8000 ppm, the use of lime is known to cause more expansion than without it. In this case, current practice forbids the use of lime to stabilize expansive soil (TxDOT 2005).
Polymer and acid stabilizers are, however, not found effective in obtaining long-term stabilization. The use of GGBFS with lime is reported relatively effective. However, the disadvantage of the use of GGBFS is its dosage, up to 5% of soil. GGBFS is a byproduct of steel plants and it is not easily accessible for many construction sites. Furthermore, the use of GGBFS is competing with the concrete industry for the limited resources.
In addition, there is currently no existing effective remedying measure for soils and concrete structures affected by sulfate attack or delayed ettringite formation (DEF). A possible method to deal with this distress is to improve the impermeability of soils and engineering structures by sealing them. However, as water molecules in ettringite are in a crystalline form, the reduction of relative humidity by sealing will not be able to remove them from the crystals of ettringite and similar minerals. As such, the sealing approach used for alkali-silica reaction (ASR) does not apply to DEF. However, in this invention, a different but effective approach is developed, as the remedying/retrofitting method, which is converting ettringite or similar hydrated minerals to crystals with less/or no water molecules in the crystal unit and thus reducing the volume.
It was reported that sodium fluosilicate as a secondary additive can accelerate strength development of clayey silt that is stabilized by polyphosphoric acid and other acidic phosphorus compounds (Michaels and Tausch 1962). New setting and hardening accelerator for hydraulic binders has been patented recently, as described in U.S. Pat. No. 6,540,826, where water-soluble fluoride comprising aluminum salts were used to prevent the later formation of ettringite so that acceleration of setting and hydration of hydraulic binders can be achieved. Another approach to decrease the tendency of soils to form ettringite is the application of barium containing compound to react with sulfate in soil, as described in U.S. Pat. No. 5,122,012. U.S. Pat. No. 6,818,057 describes ettringite formation retarder containing at least one of phosphono alkyl carboxylic acid and a salt of a phosphono alkyl carboxylic acid in hydrated calcium sulfoaluminate cements, while U.S. Pat. No. 4,992,103 described cementitious compositions that gain high compressive strength.
The conventional lime stabilization has long been proven an effective practice to improve soil properties. This invention can extend the application of lime to high sulfate soils. We describe a method of preventing, eliminating, or reducing the undesirable heave/distresses of sulfate-rich construction soil layers and expanding/cracking of concrete construction materials, with applications to new construction sites and to sites where remedying/retrofitting existing or potential distresses in subgrade, base, subbase, or construction materials is needed.